1. Field of the Invention
The present invention relates in general to the field of electronics and lighting, and more specifically to a system and method for providing ballast for light sources, such as light emitting diode light sources.
2. Description of the Related Art
Commercially practical incandescent light bulbs have been available for over 100 years. However, other light sources show promise as commercially viable alternatives to the incandescent light bulb. Light Emitting Diodes (“LEDs”) are becoming particularly attractive as main stream light sources in part because of energy savings through high efficiency light output, long life, and environmental incentives such as the reduction of mercury.
LEDs are semiconductor devices and are best driven by direct current. The brightness of the LED varies in direct proportion to the current flowing through the LED. Thus, increasing current supplied to an LED increases the brightness of the LED and decreasing current supplied to the LED dims the LED.
Dimming a light source saves energy when operating a light source and also allows a user to adjust the brightness of the light source to a desired level. Many facilities, such as homes and buildings, include light source dimming circuits (referred to herein as “dimmers”).
FIG. 1 depicts a lighting circuit 100 with a conventional dimmer 102 for dimming incandescent light source 104 in response to inputs to variable resistor 106. The dimmer 102, light source 104, and voltage source 108 are connected in series. Voltage source 108 supplies alternating current at line voltage Vline. The line voltage Vline can vary depending upon geographic location. The line voltage Vline is typically 120 Vac with a typical frequency of 60 Hz or 230 Vac with a typical frequency of 50 Hz. Instead of diverting energy from the light source 104 into a resistor, dimmer 102 switches the light source 104 off and on many times every second to reduce the total amount of energy provided to light source 104. A user can select the resistance of variable resistor 106 and, thus, adjust the charge time of capacitor 110. A second, fixed resistor 112 provides a minimum resistance when the variable resistor 106 is set to 0 ohms. When capacitor 110 charges to a voltage greater than a trigger voltage of diac 114, the diac 114 conducts and the gate of triac 116 charges. The resulting voltage at the gate of triac 116 and across bias resistor 118 causes the triac 116 to conduct. When the current I passes through zero, the triac 116 becomes nonconductive, i.e. turns ‘off’. When the triac 116 is nonconductive, the dimmer output voltage VDIM is 0 V. When triac 116 conducts, the dimmer output voltage VDIM equals the line voltage Vline. The charge time of capacitor 110 required to charge capacitor 110 to a voltage sufficient to trigger diac 114 depends upon the value of current I. The value of current I depends upon the resistance of variable resistor 106 and resistor 112. Thus, adjusting the resistance of variable resistor 106 adjusts the phase angle of dimmer output voltage VDIM. Adjusting the phase angle of dimmer output voltage VDIM is equivalent to adjusting the phase angle of dimmer output voltage VDIM. Adjusting the phase angle of dimmer output voltage VDIM adjusts the average power to light source 104, which adjusts the intensity of light source 104. The term “phase angle” is also commonly referred to as a “phase delay”. Thus, adjusting the phase angle of dimmer output voltage VDIM can also be referred to as adjusting the phase delay of dimmer output signal VDIM. Dimmer 102 only modifies the leading edge of each half cycle of voltage Vline.
FIG. 2 depicts the dimmer output voltage VDIM waveform of dimmer 102. The dimmer output voltage VDIM fluctuates during each period from a positive voltage to a negative voltage. (The positive and negative voltages are characterized with respect to a reference to a direct current (DC) voltage level, such as a neutral or common voltage reference.) The period of each voltage sine wave 202.0 through 202.N is the same as 1/frequency of Vline (FIG. 1), where N is an integer. The dimmer output voltage VDIM is a phase modulated dimmer signal. The dimmer 102 chops the voltage sine waves 202.0 through 202.N to alter the duty cycle of each sine wave 202.0 through 202.N. The dimmer 102 chops the positive half of sine wave 202.0 at time t1 so that the positive portion of sine wave 202.0 is 0 V from time t0 through time t1 and has a positive voltage from time t1 to time t2. The difference in time between time t0 at which a full cycle of sine wave 202 would have started but for the chopping and the time t1 at which dimmer output voltage VDIM is chopped introduces a phase delay α into sine wave 202. For example, for sine wave 202.0, α202.0=sine wave chop time t1-full cycle start time t0 and α202.N=t4−t3. The phase delay α is the same for the negative half of sine wave 202. Additionally, since the dimmer output voltage VDIM is periodic, the phase delay can also be referred to as a phase angle. Each half cycle of sine wave 202 is 180 degrees, and the phase angle is 180 degrees minus the (phase delay/the half period) times 180 degrees. For example, the phase angle □202.0 equals 180·[1−(t1−t0)/(t2−t0)] for sine wave 202.0 The “Sine wave 202” represents all sine waves 202.0 through 202.N. The light source 104 is, thus, turned ‘off’ from times t0 through t1 and turned ‘on’ from times t1 through t2. Dimmer 102 chops the negative half of sine wave 202.0 with the same timing as the positive half Equation [1] represents the duty cycle of dimmer 102:
                              Duty          ⁢                                          ⁢          Cycle                =                                            (                                                t                  2                                -                                  t                  1                                            )                                      (                                                t                  2                                -                                  t                  0                                            )                                .                                    [        1        ]            
When the resistance of variable resistance 106 is increased, the duty cycle of dimmer 102 decreases. Between time t2 and time t3, the resistance of variable resistance 106 is increased, and, thus, dimmer 102 chops the full cycle 202.N at later times in the first half cycle 204.N and the second half cycle 206.N of the full cycle of voltage sine wave 202.N with respect to voltage sine wave 202.0. Dimmer 102 chops the first half cycle 204.N with the same timing as the second half cycle 206.N. So, the duty cycles of each half cycle of voltage sine wave 202.N are the same. Thus, the full duty cycle of dimmer 102 for voltage sine wave 202.N is:
                              Duty          ⁢                                          ⁢          Cycle                =                                            (                                                t                  5                                -                                  t                  4                                            )                                      (                                                t                  5                                -                                  t                  3                                            )                                .                                    [        2        ]            
Since times (t5−t4)<(t2−t1), less average power is delivered to light source 104 by the sine wave 202.N than sine wave 202.0 of dimmer voltage VDIM, and the intensity of light source 104 decreases at time t3 relative to the intensity at time t2.
The voltage and current fluctuations of conventional dimmer circuits, such as dimmer 102, can destroy LEDs. U.S. Pat. No. 7,102,902, filed Feb. 17, 2005, inventors Emery Brown and Lodhie Pervaiz, and entitled “Dimmer Circuit for LED”(referred to here as the “Brown Patent”) describes a circuit that supplies a specialized load to a conventional AC dimmer which, in turn, controls an LED device. The Brown Patent describes dimming the LED by adjusting the duty cycle of the voltage and current provided to the load and providing a minimum load to the dimmer to allow dimmer current to go to zero.
Exemplary modification of leading edges and trailing edges of dimmer signals is discussed in “Real-Time Illumination Stability Systems for Trailing-Edge (Reverse Phase Control) Dimmers” by Don Hausman, Lutron Electronics Co., Inc. of Coopersburg, Pa., U.S.A., Technical White Paper, December 2004, which is incorporated herein by reference.
Line voltage fluctuations adversely affect LEDs. Line voltage fluctuations can produce a disproportional change in drive current to the LED. Increases in drive current increase heat and, thus, reduce the useful life of the LED. Useful life can be defined, for example, when the light output of the LED declines by thirty percent or more.